Cathode coating for electrical discharge devices and method for making the same



Nov. 15, 1955 H. HElNE ET AL 2,724,070

T. CATHODE COATING FOR ELECTRICAL DISCHARGE DEVICES AND METHOD FOR MAKING THE SAME Filed Feb. 9, 1953 INVENTORS and g Gig/P65 Mi /6727?.

ATTORN Ce r 2,724,070

Patented Nov. 15, 1955 CATHODE COATING FOR ELECTRICAL DIS- CHARGE DEVICES AND METHOD FOR MAKING THE SAME Thomas H. Heine, Cedar Grove, and George Meister,

Newark, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 9, H53, Serial No. 335,908 9 Claims. (Cl. 313-437) The present invention relates to electron emissive coatings for cathodes of electrical discharge devices and, more particularly, to an electron emissive coating for the fused type of cathode which may be used with instant start fluorescent lamps.

Normally, when starting a fluorescent lamp, it is necessary to preheat the cathodes in order to prevent premature failure of the cathode due to loss of emission material from ion and electron bombardment. Thus, there is normally a time lag between turning on an ordinary hot cathode fluorescent lamp and the application of starting voltage between the electrodes because of the necessity of heating the electrodes before application of the starting voltage across them. In order to eliminate this start ing time lag for fluorescent lamps and yet achieve long cathode life, a fused cathode type fluorescent lamp may be used in which the starting voltage is applied directly to the unheated cathodes, thus giving instant start.

Fused cathodes are disclosed in Patent No. 2,476,590 to George S. Evans, dated July 19, 1949, and assigned 'to the same assignee as the present invention, in which the method for making fused cathodes is carefully outlined. The fused cathode emission material therein described is barium oxide, and the life of a fluorescent lamp with a barium oxide fused cathode is greately increased over a lamp utilizing an unfused cathode when used in applications where frequent instant starting is required, such as a flashing sign. However, the life of such fused cathode fluorescent lamps under the more normal instant start burning, such as a test cycle of three hours on, twenty minutes off, is only as good as, or slightly better than, standard non-fused cathode fluorescent lamps, operating under the same conditions, with the starting voltage applied directly to the unheated cathode. Some improve ment in life has been obtained by use of barium oxide and zirconium dioxide as a fused cathode coating material, but results have been only slightly better than the standard unfused type or the previous fused type of cathode for fluorescent lamps when used under the same conditions. It is the general object of our invention to avoid and overcome the foregoing difiiculties of and objections to prior art practices by providing an electron emissive material adapted for use as a cathode coating in electrical discharge devices, which coating will have a long life.

Another object of our invention is to provide an electron emissive material adapted for use as a cathode coating in instant start fluorescen lamps, which coating will have a long life.

A still further object of our invention is to provide an improved method of preparing cathode coatings of electron emissive materials for electrical discharge devices.

The aforesaid objects of our invention, and other objects which will become apparent as the description pro- .ceeds, are achieved by providing a cathode electron emissive coating which is composed of barium oxide and magnesium oxide, with or without zirconium dioxide.

Our invention will be better understood by reference to the following description of preferred embodiments of our invention and by reference to the accompanying drawing in which Fig. l is a cut-away View of an instant start fluorescent lamp having a fused cathode utilizing our new emission material;

Fig. 2 is a sectional view of an indirect heated type of cathode utilizing the fused emission material of this invention.

Although the principles of our invention are broadly applicable to electron emissive coatings. for cathodes of electrical discharge devices, the invention is usually empioyed in conjunction with a fused type of cathode used with an instant start fluorescent lamp, and hence it has been so illustrated and will be so described.

The accompanying drawing illustrates the use of our emission material in a fused cathode fluorescent lamp 10. The lamp 10 has a light transmitting envelope 12 with bases 14 at each end of the envelope. A cold-hot cathode or electron emissive coated electrode 16 is supported at each end of the envelope by lead-in conductors 18 sealed through a re-entrant stem press 20 and which make electrical connection to the base 14, thereby providing means for supplying power to the lamp. This type of cathode is called a cold-hot" cathode, since the cathode is started cold and operated hot. In the preferred embodiment of our invention the cold-hot cathodode is a filamentary type cathode in which the refractory metal wire is wound into a minor coil, which in turn is wound into a major coil, thus forming a coiled-coil as is common in the art. This coiled-coil refractory metal wire is preferably tungsten, but may be any suitable refractory metal, such as molybdenum. Fused on the inner coil of this coiled-coil refractory metal filament and adhering thereto is the emission material of our invention. The sealed light transmitting envelope 12 encloses a small amount of mercury 22 to provide mercury vapor, and a filling of inert ionizable gas, such as argon, to facilitate starting. A representative pressure of inert ionizable gas is 3.2 mm., and a representative operating mercury vapor pressure is 10 microns. A fluorescent material 24 is coated on the inner side of envelope 12.

Fused cathodes used in instant start fluorescent lamps generally utilize alkaline earth oxides either singly or in combination with one or both of the other alkaline earth oxides. The alkaline earth metal of the cathode is preferably barium and the cathode is made in accordance with the teachings of the heretofore-mentioned patent to George S. Evans. In making a fused cathode of barium oxide, a coating of barium carbonate or some other compound of barium which will reduce to the oxide is applied to the base metal of the cathode. The application to the base metal may be accomplished by first ball milling the barium carbonate to a small particle size, mixing the carbonate with a binder, and spraying, painting or otherwise applying the coating to the base metal. Upon heating, the decomposition of barium carbonate starts at about 500 C. However, in the fusion treatment the temperature is quickly raised to liOO" C. so that the carbonate has small chance to totally or even appreciably decompose due to heating before 1100" C. is reached. Thus the carbonate is heated from 500 C. to 1100 C. within 30 seconds and preferably within 20 seconds, when heating an indirect heated type cathode as disclosed in the heretofore-mentioned patent to George S. Evans. In the case of a cold-hot cathode, as used in the preferred embodiment of our invention, this heating time may be reduced to 10 seconds, since the refractory metal wire is in direct contact with the barium carbonate and more rapid heating is possible.

According to the present invention, the fused electron emissive coating of the cathode contains magnesium oxide in addition to the barium oxide and may contain an addition of zirconium dioxide. The minimum amount of barium oxide which may be used in our fused electron emissive coating is controlled by the minimum amount of barium carbonate which may be used in the original unfused coating mixture, and which is reduced to barium oxide upon fusion. It has been found that if there is less than 85 parts by weight of barium carbonate in the unfused mixture, the method of heat treatment hereinafter outlined will not result in a fused electron emissive coating such as is disclosed in the heretofore-mentioned patent to George S. Evans. Thus, not less than 85 parts by weight of barium carbonate should be used in the original mixture, if fusion is to be effected. The parts by weight of magnesium oxide in the unfused coating mixture may vary from 1 to 15. The parts by weight of zirconium dioxide in the unfused coating mixture may vary from to 14. Of course, the totalparts by weight of the magnesium oxide plus the zirconium dioxide in the unfused coating mixture should not exceed 15 After fusion, when the barium carbonate is reduced to barium oxide, the resultant percentages of barium oxide, magnesium oxide and zirconium dioxide will fall within the limits shown in the following tables:

Coating mixture before fusion I Permissible parts Component; by weight Barium carbonate 85-99 Magnesium oxide 115 Zirconium dioxide 014 Magnesium oxide plus zirconium dioxide not to exceed 15 parts by weight.

Fused electron emissive coating Permissible parts Component: by weight Barium oxide 81.5-98.5 Magnesium oxide 1.2-185 Zirconium dioxide 0 17.3

Magnesium oxide plus zirconium dioxide not to exceed 18.5 parts by weight.

Representative weights of the amount of fused electron emissive coating which may be used on each cathode will vary depending upon the application, but for a 40 watt instant start fluorescent lamp, 13 to 18 mg. of fused coating material per cathode will be satisfactory.

A fused cathode electron emissive coating which gives very good results in life tests consists of 87.5 parts or percent by weight barium oxide, 6.25 parts or percent by weight magnesium oxide, and 6.25 parts or percent by weight zirconium dioxide. To have a cathode emissive coating which contains these percentages of fused material, the base metal of the cathode may be coated with a material composed of, by weight, 90 parts barium carbonate, parts magnesium oxide, and 5 parts zirconium dioxide. This coating material may be prepared and applied by ball milling a mixture of the carbonate and oxides into small particles, mixing with a binder as is well known in the art, and applying the resultant mixtures to the base metal by painting, spraying, or dipping. The cathode is then heat treated in the same manner as de-' scribed heretofore for making fused cathodes of barium oxide; While we prefer to utilize barium carbonate, magnesium oxide, and zirconium dioxide as the batch cornponents of our cathode coating, any other compounds may beused which will decompose to the oxides of barium, magnesium and zirconium respectively, upon heating. Preferably the fused coating adheres to the inner coil only, and does not bridge the outer coils of the coiled-coil.

In order to test our new emission material for fused cathodes, we incorporated the material containing 87.5%

by weight of barium oxide, 6.25% by weight of magnesium oxide, and 6.25% by weight of zirconium dioxide into 40 watt instant start fluorescent lamps. The lamps were then tested on a test cycle which consisted of in stant start burning of three hours on and twenty minutes off. Control lamps had cathodes Whose emission material consisted of 87.5% by weight barium oxide, and 12.5% by weight zirconium dioxide, which is the fused cathode coating which has heretofore been the most satisfactory with respect to life. The average life of the cathodes in the control lamps was approximately 4700 hours, while the average life of the cathodes in the lamps which embodied our new emission material was approximately 6882 hours, an increase of 46%.

It will be seen from the above that We have provided a cathode for an instant start fluorescent lamp which has a long life and that we have also provided anemission material which is adaptable for use in electrical discharge devices which will give long cathode life.

While the preferred embodiment of our invention has been illustrated and described as relating to a. cold-hot cathode, as pertaining to fluorescent lamps, it is obvious that our method and coating may be utilized with an indirect heated type cathode, as is normally used Withradio tubes. In such an application, the base metal of the cathode would consist of nickel or an alloy of nickelcobalt and ferrotitanium. Such an indirect heated type of cathode is illustrated in Fig. 2 and is more fully described in Patent No. 2,429,291 to Okress.

While we have described a specific embodiment of our invention, it will be understood that modifications may be made within the spirit and scope of our invention.

We claim:

1. A cathode for an electrical discharge device com prising a refractory metal coil and a fused coating there on and adhering thereto, said coating comprising barium oxide and magnesium oxide, said barium oxide being not less than 81.5% nor more than 98.5% by weight of said coating.

2. A cold-hot cathode for an electrical discharge device comprising a coiled-coil refractory metal wire and a fused coating thereon and adhering thereto, said coating comprising barium oxide and magnesium oxide, said barium oxide being not less than 81.5 nor more than 98.5% by weight of said coating.

3. A cathode for an electrical discharge device comprising a refractory metal wire and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and an addition of zirconium dioxide if desired, said barium oxide being not less than 81.5 nor more than 98.5% by weight of said coating, said magnesium oxide being not less than 1.2% nor more than 18.5% by weight of said coating, said zirconium dioxide being not more than 17.3% by weight of said coating, and the additive weights of said magnesium oxide and said zirconium dioxide not exceeding 18.5% by weight of the said coating.

'4. A cold-hot cathode for an electrical discharge device comprising a coiled-coil tungsten wire and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and an addition of zirconium dioxide if desired, said barium oxide being not less than 81.5% nor more than 98.5% by weght of said coating, said magnesium oxide being not less than 1.2% nor more than 18.5% by weight of said coating, said zirconium dioxide being not more than 17.3% by weight of said coating, and the additive weights of saidmagnesium oxide and said zirconium dioxide not exceeding 18.5% by weight of the said coating.

5. A cathode for an electrical discharge device comprising a coiled-coil molybdenum wire and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and an addition of zirconium dioxide if desired, said barium oxide being not less than 81.5 nor more than 98.5 by weight of said coating, said magnesium oxide being not less than 1.2% not more than 18.5% by weight of said coating, said zirconium dioxide being not more than 17.3% by weight of said coating, and the additive Weights of said magnesium oxide and said zirconium dioxide not exceeding 18.5 by weight of the said coating.

6. A cathode for an electrical discharge device comprising a coiled-coil tungsten wire and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and zirconium dioxide, said barium oxide being about 87.5% by weight of said coating, said magnesium oxide being about 6.25% by weight of said coating, and said zirconium dioxide being about 6.25% by weight of said coating.

7. An indirect heated type cathode comprising a base portion of nickel and a fused coating thereon and adhering thereto, said coating comprising barium oxide and magnesium oxide, said barium oxide being not less than 81.5% nor more than 98.5% by weight of said coating.

8. An indirect heated type cathode comprising a base portion of nickel and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and an addition of zirconium dioxide, if desired, said barium oxide being not less than 81.5% nor more than 98.5% by weight of said coating, said magnesium oxide being not less than 1.2% nor more than 18.5% by weight of said coating, said zirconium dioxide being not more than 17.3% by weight of said coating, and the additive weights of said magnesium oxide and said zirconium dioxide not exceeding 18.5% of said coating.

9. An indirect heated type cathode comprising a base portion of an alloy of nickel-cobalt and ferrotitanium and a fused coating thereon and adhering thereto, said coating comprising barium oxide, magnesium oxide and an addition of zirconium dioxide if desired, said barium oxide being not less than 81.5% nor more than 98.5% by weight of said coating, said magnesium oxide being not less than 1.2% nor more than 18.5% by weight of said coating, said zirconium dioxide being not more than 17.3% by weight of said coating, and the additive weights of said magnesium oxide and said zirconium dioxide not exceeding 18.5% by weight of said coating.

by weight References Cited in the file of this patent UNITED STATES PATENTS 1,830,825 Cooper et a1. Nov. 10, 1931 2,018,993 Braselton Oct. 29, 1935 2,476,590 Evans July 19, 1949 2,530,394 Lowry et a1 Nov. 21, 1950 2,542,352 Peters Feb. 20, 1951 FOREIGN PATENTS 624,009 Great Britain May 26, 1949 OTHER REFERENCES The Oxide Coated Cathode, by Dr. P. S. Wagener, Chapman and Hall, London, 1951, vol. II, pgs. 221-223. 

7. AN INDIRECT HEATED TYPE CATHODE COMPRISING A BASE PORTION OF NICKEL AND A FUSED COATING THEREON AND ADHERING THERETO, SAID COATING COMPRISING BARIUM OXIDE AND MAGNESIUM OXIDE, SAID BARIUM OXIDE BEING NOT LESS THAN 81.5% NOR MORE THAN 98.5% BY WEIGHT OF SAID COATING. 